This invention relates to a process for removing compounds containing silicon from hydrocarbon streams and is particularly concerned with a process for removing organosilicon compounds from reformer feedstocks to prevent silicon poisoning of the reformer catalyst.
Catalytic reforming is a conventional refining process which is utilized for such purposes as dehydrogenation, hydrogenation, cyclization, dehydrocyclization, isomerization and dehydroisomerization of selected hydrocarbons. Catalytic reforming is normally utilized to upgrade straight run or cracked naphtha feedstocks by increasing the octane number of the feedstock's gasoline fraction. In a typical reforming process in which a straight run or cracked naphtha is upgraded, the feedstock is contacted with a catalyst comprising a noble metal on alumina. The conditions utilized in the reforming process will vary depending upon, among other factors, the type of feed being processed and the desired increase in octane level.
Reforming catalysts, particularly those containing platinum, and most particularly those comprising platinum, rhenium and chlorine, are poisoned or deactivated rapidly in the presence of sulfur components. Thus, to achieve maximum run lengths and increase process efficiency, it is necessary to reduce the sulfur content of reformer feedstocks as low as possible.
One common method of removing sulfur compounds from reformer feedstocks is to subject the feedstock to catalytic hydrodesulfurization by contacting the feedstock with molecular hydrogen in the presence of a sulfur-tolerant hydrotreating catalyst. The sulfur compounds in the hydrocarbon stream are converted to hydrogen sulfide, which may be separated from the hydrocarbon stream by conventional means prior to subjecting it to reforming. Although highly effective sulfur removal may be achieved by catalytic hydrodesulfurization, the efficiency of the process is ultimately limited by equilibrium and/or kinetic considerations. In general, it is not possible to obtain hydrodesulfurized products containing less than about 0.5 ppmw sulfur as is desired in most reforming operations. Furthermore, it is impossible to guard against upsets in the catalytic hydrodesulfurization units which can result in high levels of organosulfur compounds remaining in the reformer feedstock.
In addition to being highly sensitive to sulfur components, reforming catalysts are also poisoned by compounds containing silicon. One common source of hydrocarbon streams containing silicon compounds is the delayed coking unit utilized in many petroleum refineries. Such a unit is used to convert residual oils into more valuable products. The overhead vapors from the coking drum, which is part of the delayed coking unit, are normally fractionated into various cuts including a gasoline boiling range stream commonly referred to as coker gasoline or coker naphtha. This stream generally possesses a low octane number and is therefore unsuitable for use as automotive fuel without upgrading. Thus, it has become common practice to increase the octane number of coker gasoline by subjecting it to catalytic reforming. The coker gasoline will not only contain sulfur compounds but, quite frequently, will contain organosilicon components derived from silicon defoamers, such as polydimethyl siloxanes, added in the delayed coking process to prevent foaming.
In view of the above, it is desirable to remove both sulfur compounds and silicon compounds from coker gasoline and other hydrocarbon streams that are to be processed in catalytic reformers. If a stream containing both sulfur and silicon compounds is subjected to catalytic hydrodesulfurization or hydrotreating, the sulfur will not only be removed by conversion to hydrogen sulfide but the silicon compounds will deposit on the catalyst. It is, however, not desirable to use the hydrotreating catalyst to remove silicon components from reformer feedstreams. After a certain amount of silicon deposits on the catalyst, complete removal of silicon compounds will no longer take place and the effluent from the hydrotreating unit will contain at least trace amounts of organosilicon components which will irreversibly poison the reforming catalyst. Moreover, the deposited organosilicon components will have a deleterious effect on the hydrotrating catalyst, tending to poison it and decrease its effectiveness.
Accordingly, it is one of the objects of the present invention to provide a process for removing silicon components from reformer feedstreams prior to subjecting the feedstreams to catalytic hydrotreating to remove sulfur components. It is another object of the invention to provide a process for removing silicon components from reformer feedstreams which have previously been subjected to catalytic hydrotreating. Altenatively, it is a further object of the invention to provide a process which can be used to simultaneously remove sulfur and silicon components from reformer feedstreams. These and other objects of the invention will become more apparent in view of the following description of the invention.